The present invention concerns the abrasionproof surface treatment by laser of a mechanical part. More particularly, the present invention concerns the surface treatment of a sintered mechanical part obtained by powder metallurgy by laser deposit of a cermet coating, the cermet being a composite material formed by ceramic products coated in a metallic binder. The present invention also concerns a manufacturing method of such a mechanical part.
The coatings composed of spherical tungsten carbides in a nickel-chrome matrix and deposited by laser on cast irons or on traditional steel and thus, non sintered, already exist in the prior art. An example of this type of coating is described as an example in the Canadian patent application No. 2,126,517. The laser deposit is a coating technique that enables to deposit thick layers of very hard material on the surface of a metallic part. A continuous CO2 laser delivers an infrared beam whose energy is used to superficially melt the base metal to be coated as well as the filler metal brought in the form of fine powder. A coaxial nozzle traversed in its centre by a laser beam enables the arrival and the injection of powders forming the coating, the latter resembling to a welding cord. To this day, this type of laser deposit has only been used to coat non sintered traditional metallic parts, used more particularly in very abrasive conditions.
It is well known in the prior art that the mechanical parts manufactured by powder metallurgy do not possess the physical characteristics to work in tension, in abrasion or in friction and this is due to the presence of a high number of pores in the surface of these sintered parts, thus decreasing the initiation period of the cracks in comparison to a forged or machined part. Thus, the porosity in the surface of the parts manufactured by powder metallurgy prevents the production of mechanical parts able to resist to shock and/or abrasive wear because of the brevity of the initiation period of the cracks.
An object of the present invention is to propose a sintered mechanical part obtained by powder metallurgy and offering a very high resistance to shock, to abrasion and to friction, as well as a very good mechanical resistance of the body of the part.
More particularly, the object of the present invention is to propose a mechanical part with abrasionproof surface characterized in that it comprises:
a sintered metallic body obtained by powder metallurgy; and
a cermet coating covering the metallic body and having an external surface constituting the abrasionproof surface, the mechanical part being characterized in that:
said coating is obtained by laser deposit by coaxially injecting in a laser beam a flux of a mixture of metallic powders and ceramic powders containing spheroidal-shaped carbides, said mixture being intended to form said coating, which is characterized in that it is exempt of porosity, is metallurgically bound to the metallic body, has a thickness ranging from 10 microns to 1 mm and comprises spheroidal-shaped carbides in a metallic matrix.
A man of the art will understand that xe2x80x9cmetallurgically bound to the metallic bodyxe2x80x9d, means that the coating is fused to the surface of the sintered part, the microstructure at the base of the coating being intimately linked to the microstructure of the body of the part.
The mechanical part may comprise any part traditionally used in very abrasive conditions or in high tension, for example, the barking tools mounted on the barking arms.
The object of the present invention is also to propose a method for manufacturing the mechanical part described above. More particularly, the method is characterized in that it comprises the following steps:
a) providing a sintered mechanical part obtained by powder metallurgy; and
b) depositing by laser process a cermet coating on an external surface of said mechanical part.
The laser process of deposit comprises, preferably, the following steps:
guiding a laser beam on the external surface of the part, the laser beam releasing a certain temperature and fusing a certain thickness of said external surface;
injecting in the laser beam a constant flux of a mixture of ceramic powders and of metallic powders intended to form the cermet coating, the ceramic powders having a higher fusion temperature than the temperature of the laser beam and the metallic powders having a lower fusion temperature than the temperature of the laser beam, so that the laser fuses the metallic powders of the powder mixture that is deposited on the external surface of the part; and
displacing the laser beam relative to the mechanical part to thus sweep the external surface and form the cermet coating.
The powder mixture can be injected in the laser beam by means of a coaxial nozzle traversed in its centre by the laser beam, the nozzle allowing the arrival of the powder mixture and its injection in the laser beam.
The laser beam is, preferably, fixed and the mechanical part is installed on a mobile table movable relative to said laser beam.
The coating according to the present invention being deposited by laser enables the surface of the sintered part to be coated to melt under the effect of the laser beam. The surface of the sintered part to be covered is thus fused on a thickness ranging from 10 xcexcm to 1 mm, which allows the closing of the pores on the surface, typical of sintered parts and, consequently, the increase of its resistance to shock. Moreover, the small surface covered at a given instant by the laser allows the self-hardening of the exposed zone, following the displacement of the beam, by heat-sink effect of the surrounding metallic volume. The coating obtained according to the present invention offers also a very low porosity because of the complete fusion of the filler metallic powders during their travel through the laser beam.